When an oil or gas well is drilled, it is common to run and cement a casing across the hydrocarbon bearing formations to form a pressure tight and safe method of controlling the formation pressures. During the subsequent completion phase of the well the casing is perforated to allow oil to flow. Typically, the casing is perforated using shaped charges to create a controlled explosion which blast multiple small holes through the casing called perforations. Burrs are generally created as a result of creating such perforations through the casing wall. A burr is a raised edge or comprises small pieces of material that remain attached to the edge of the perforation after the process of creating perforations is complete; burrs are generally sharp and they protrude into the wellbore. As such, if burrs remain they can cause damage to subsequent insertions to the casing, for example production screens or packers.
Following the perforation process the next stage of the completion phase is often to run packer assemblies into the casing. Packers are located in the casing in a position that straddles the perforated areas and provide means to control the flow of hydrocarbons into the wellbore.
A packer is designed to grip and seal against the inside diameter of the pipeline or well casing in which the tool is deployed. A sealing function is provided by sealing elements, which are generally composed of flexible elastomeric material, for example rubber. It will be appreciated that the elastomeric material may be vulnerable to damage or tearing if burrs remain in the casing. If torn or damaged by the burrs the ability to seal may be diminished and leakage is likely to occur. This may result in costly remedial operations.
To prevent such damage it is common to run a string mill assembly or a scraper into the casing to dress the casing or remove any perforation burrs.
Tools have been designed which incorporate the aggressiveness of a mill with the flexibility of the scraper such that the scraper blocks include milling media to remove the perforation burrs. Unfortunately, in such tools the blades can be too aggressive, where dressing the perforations may be performed as a first stage operation, but continued movement of the tool after milling is complete often causes unwanted damage to the casing.
U.S. Pat. No. 7,191,835 describes a milling tool which can be disengaged from the casing wall. The tool incorporates a series of milling blades which are supported on springs and where the milling blades are biased into contact with the casing wall such that movement of the tool removes burrs. The blades are retractable, where the blades are moved inside the tool body by the action of dropping a ball onto a ball seat and increasing fluid pressure behind the ball until a shear pin shears to allow downward movement of a supporting sleeve and therefore re-positioning of the springs such that the blades are no longer supported in an extended position. From this position the blades are retractable and no longer in contact with the casing wall.
U.S. Pat. No. 8,141,627 also describes an example of a tool that includes retractable milling blades operable to clean inside a casing when the mill blades are extended and to prevent any cutting action when the blades are retracted.
Both examples described briefly above may be effective as deburring tools. However, the retractable nature of each tool means there is typically a space between the blades and the operating sleeve. It will be appreciated that this space is susceptible to filling with debris, which can prevent the blades from retracting. Neither system provides any feedback to an operator. Therefore, as a result an operator may continue with operations unaware that the tool is still extended, or at least is not fully retracted. Continued operation of the tool may result in damage to the casing. Such damage may be as detrimental as the burrs to subsequently deployed components.
It is desired to provide an improved deburring tool.